The present invention relates to a method and a device for pointing a fixed antenna having a reflector including at least one transceiver source capable of aiming at a plurality of satellites situated between two extreme positions S1, S2 on a geostationary orbit.
The method and device can also be implemented with an antenna having a source whose radiation pattern is steerable so as to be able to select a selected incident beam from a plurality corresponding to different target orbital positions. It can also be implemented with an antenna provided with a source that is motor-driven along the focal line. In addition, the antenna can be of the following types:
an antenna with a reflector in a centered configuration having a focal line comprising as many sources as there are target orbital positions; and
an antenna having a reflector in an offset configuration having a focal line comprising as many sources as there are target orbital positions.
Such antennas are described in document FR 2 746 218, for example, which discloses a support for mounting two converters (transmitter or receiver heads) on a parabolic antenna, and document FR 2 701 337 describes a support for a plurality of receiver heads on a parabolic antenna.
The invention also applies to an array antenna with azimuth scanning.
FIGS. 1, 2, and 3 illustrate a known method of pointing a multi-satellite antenna having a centered-configuration reflector with a plurality of sources on its focal line. This pointing consists in aiming at the intermediate orbital position SM where a middle satellite is located, halfway angularly between the extreme positions S1 and S2. For this purpose, the antenna is turned about an azimuth axis A and about an elevation axis B as shown in FIG. 4. To align the reflector and the plane containing the sources on the plane OS1S2, the antenna is turned about its own axis pointing at the orbital position SM through a roll angle i, and then the spacing between the sources is adjusted so as to receive all of the orbital positions.
The principle on which such an antenna receives is illustrated in FIGS. 2 and 3 which show two intermediate target orbital positions S3 and S4 situated between the two extreme orbital positions s1 and s2. The idea is to align the transceiver sources s1, s2, s3, s4, and sm on the positions S1, S2, S3, S4, and SM. Error calculation shows that if the antenna is accurately aligned on S1 and S2, then the error on S3, S4, and Sx is relatively small and, to a first approximation, it is possible to assume that the orbital positions S1, S2, S3, S4 and SM all line in the same plane OS1S2.
The above-described prior art pointing method works providing it is possible to rely on the orbital position SM. Unfortunately, there need not be any satellite in position SM. Under such circumstances, it is necessary to use an existing satellite that is close to the position SM e.g. S3 for pointing purposes. This can be done by offsetting the source S3 by an amount corresponding to the angle S3OM and then applying the above-described pointing technique. This provides approximate pointing on S1, S2, S3, and S4. Under such circumstances, as can be seen in FIG. 5, the roll axis (C) corresponding to the roll angle is the axis extending towards the virtual satellite in orbital position SM.
However, because the elevation corresponding to satellite S3 is different from that corresponding to satellite SM, pointing requires successive readjustments due in particular to how the roll angle is applied. In addition, this pointing is never optimal over all positions simultaneously. Furthermore, because the primary pointing is on a position other than SM, pointing errors and elevation errors accumulate thus making it impossible to align S1 and S2 with a single roll angle starting from an intermediate position that is not in the middle. Consequently, the roll angle cannot satisfy both pointing on S1 and on S2, particularly when S3 is far from SM.
The object of the invention is to provide a method and a device enabling the above-described drawbacks of the prior art to be mitigated.
The method of the invention is characterized in that it comprises the steps of consisting in:
offsetting said transceiver source on the focal line of the antenna through a distance d from the middle of said focal line so as to aim at one of the extreme positions S1, S2, said distance d being determined as a function of an angle xcex1 formed between a first line connecting the origin O of the focal axis of the reflector to the target extreme position and a second line connecting said origin O to the middle position SM of the geostationary orbit;
turning the antenna through the angle xcex1 about an axis D perpendicular to the plane containing the focal line and the origin O of the focal axis; and
adjusting the roll angle by turning the antenna about its own axis pointing to the satellite situated at the target extreme position, so as to aim at the other extreme position and bring the focal line into alignment with the set of satellites situated between the positions S1 and S2.
The first and second steps described above can be performed in either order.
According to another characteristic of the method of the invention, when the antenna has a plurality of sources, the height of each of them in a plane perpendicular to the plane containing the focal line and the origin O of the focal axis of the reflector is adjusted independently of the others.
The device for implementing the method of the invention is characterized in that it comprises a mechanism for fixing the antenna to said support, said mechanism also enabling the antenna to be turned through an angle xcex1 about an axis D perpendicular to the plane containing the focal line and the origin O of the focal axis of the reflector so as to steer the reflector transversely.
By means of the method and the device of the invention, pointing on different orbital positions is performed with great accuracy.